Yu-xi Liu

Linear response of superconducting flux quantum circuits

  1. Hui-Chen Sun,
  2. Yu-xi Liu,
  3. J. Q. You,
  4. E. Il'ichev,
  5. and Franco Nori
We study the microwave absorption of a driven three-level quantum system, which is realized by a superconducting flux quantum circuit (SFQC), with a magnetic driving field applied to
the two upper levels. The interaction between the three-level system and its environment is studied within the Born-Markov approximations, and we take into account the effects of the driving field on the damping rates of the three-level system. We study the linear response of the driven three-level SFQC to a weak probe field. The susceptibility of the probe field can be changed by both the driving field and the bias magnetic flux. When the bias magnetic flux is at the optimal point,the transition from the ground state to the second excited state is forbidden and the three-level system has a ladder-type transition. Thus, the SFQC responds to the probe field like natural atomic systems with ladder-type transitions. However, when the bias magnetic flux is away from the optimal point, the three-level SFQC has Δ-type transition, thus it responds to the probe field like a combination of natural atoms with ladder-type transitions and natural atoms with Λ-type transitions. In particular, we give detailed discussions on the conditions for realizing electromagnetically induced transparency and Autler-Townes splitting in three-level SFQCs.
arxiv pdf

Feedback-induced nonlinearity and superconducting on-chip quantum optics

  1. Zhong-Peng Liu,
  2. Hui Wang,
  3. Jing Zhang,
  4. Yu-xi Liu,
  5. Re-Bing Wu,
  6. and Franco Nori
Quantum coherent feedback has been proven to be an efficient way to tune the dynamics of quantum optical systems and, recently, those of solid-state quantum circuits. Here, inspired
by the recent progress of quantum feedback experiments, especially those in mesoscopic circuits, we prove that superconducting circuit QED systems, shunted with a coherent feedback loop, can change the dynamics of a superconducting transmission line resonator, i.e., a linear quantum cavity, and lead to strong on-chip nonlinear optical phenomena. We find that bistability can occur under the semiclassical approximation, and photon anti-bunching can be shown in the quantum regime. Our study presents new perspectives for engineering nonlinear quantum dynamics on a chip.
arxiv pdf

From blockade to transparency: controllable photon transmission through a circuit QED system

  1. Yu-xi Liu,
  2. Xun-Wei Xu,
  3. Adam Miranowicz,
  4. and Franco Nori
A strong photon-photon nonlinear interaction is a necessary condition for photon blockade. Moreover, this nonlinearity can also result a bistable behavior in the cavity field. We analyze
the relation between detecting field and photon blockade in a superconducting circuit QED system, and show that the photon blockade cannot occur when the detecting field is in the bistable regime. We further demonstrate that the photon transmission through such system can be controlled (from photon blockade to transparency) by the detecting field. Numerical simulations show that our proposal is experimentally realizable with current technology.
arxiv pdf